feat(lib): add experimental _EXP_STRING backed by std::string

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/tests/library/exp-string/notes.md b/tests/library/exp-string/notes.md
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+# Trace notes
+
+Review status: pending human review.
+
+Copies of hello are independent; assigning world to the copy gives helloworld when concatenated. Empty length is zero, and three NUL bytes retain length three. One hundred appended five-byte words grow 20000 bytes to 20500, preserved by BYVAL and RETURN. C++17 formatting gives 42 and 1.250000. Parsing 12.5 compares equal to 12.5. Trailing junk and negative counts emit the explicit library reasons at user lines 27 and 32, then are caught. Discarded numeric and Echo results must release their owned strings at statement end. The explicitly deleted heap object prints heap. This fixture covers compiler/library extensions, including numeric text formatting.